In the manufacturing of a light-emitting diode (LED), an epitaxial layer is grown on a substrate. The substrate functions as a seed layer. When the lattice constant of the substrate is similar to the lattice constant of the epitaxial layer, a defect such as dislocation between the epitaxial layer and the substrate can be reduced. When the epitaxial layer is formed on the substrate, a stress is formed between the epitaxial layer and the substrate under different temperature regions of the reactor furnace. The stress affects the epitaxial quality of the epitaxial layer, and the stress may result in warp in the epitaxial layer. Thus, the material of the substrate is preferably similar to the material of the epitaxial layer. Because of the similar physical characteristics, such as the lattice constant, of the substrate and the epitaxial layer, the stress can be reduced. However, for some epitaxial layers, there is no suitable substrate available for use, neither the same material as the epitaxial layer, nor the same lattice constant as the epitaxial layer. Further, in consideration of the cost of the production, there may be no suitable substrate available.
Based on the reasons described above, once the material of the substrate and the material of the epitaxial layer are different, one or more materials of the epitaxial layer is different from the material of the substrate when the epitaxial layer is composed of a plurality of materials, or the lattice constant, the thermal expansion coefficient or the hardness of the epitaxial layer is different from that of the substrate, which results in different degrees of stress between the substrate and the epitaxial layer at different temperatures of the reactor furnace when the epitaxial layer is formed on the substrate. The stress may result in different degrees of curvature or warp. Mild stress may result in uneven heating of the epitaxial layer, which further results in poor epitaxial quality. The bending caused by the epitaxial layer warp also impacts the following process. However, if the stress is too large, the epitaxial layer may rupture.
The growth method of the epitaxial layer of the light emitting diode comprises vapor phase epitaxy (VPE) or metal organic chemical vapor deposition (MOCVD). The metal organic chemical vapor deposition (MOCVD) method is most commonly used to grow the epitaxial layer, such as GaN or AlGaInP. First, a substrate is disposed on a wafer carrier. After that, an epitaxial layer is formed on the substrate to form a wafer structure in a reactor furnace. The temperature of the reactor furnace changes continually during the formation of the epitaxial layer. Because the lattice constants or the thermal expansion coefficients of the epitaxial layer and the substrate are different from each other, the wafer structure has different degrees of curvature or warp in different temperature regions.
When the wafer structure is bowed, the wafer cannot contact with the wafer carrier closely, which results in uneven temperature distribution across the whole wafer surface. If the light-emitting layer is grown on the wafer, the light-emitting wavelength distribution range across the whole wafer is large.
FIG. 1 illustrates a conventional wafer carrier 10. The wafer carrier 10 comprises a carrier body 100 having an opening 102. A bottom surface 103 of the opening 102 is a flat surface. A wafer 104 comprises a growth substrate and an epitaxial layer grown on the growth substrate, and the epitaxial layer comprises a light-emitting layer. When the epitaxial layer is grown on the growth substrate, the furnace temperature is changed continually. Due to the lattice constants and the thermal expansion coefficients of the epitaxial layer and the growth substrate are different from each other, different degrees of curvature and warp are produced on the wafer in different temperature regions. As shown in FIG. 1, the wafer 104 is convex warp in a cross-sectional view. When the light-emitting layer is grown on the growth substrate, only partial surface of the wafer 104 is contacted with the bottom surface 103 of the opening 102 of the wafer carrier 10. When the reactor furnace temperature for the growth of the light emitting layer is set at a value by considering the condition of the center area of the wafer 104, the growth temperature of the edge of the wafer 104 is different from the growth temperature of the center of the wafer 104. Because the growth temperature varies with different regions of the wafer 104, the light-emitting wavelengths of different regions of the wafer 104 are also different.
FIG. 2 illustrates a conventional wafer carrier 20. The wafer carrier 20 comprises a carrier body 200 having an opening 202. A bottom surface 203 of the opening 202 is a flat surface. A wafer 204 comprises a growth substrate and an epitaxial layer grown on the growth substrate, and the epitaxial layer comprises a light-emitting layer. As illustrated in FIG. 2, the wafer 204 is concave warp in a cross-sectional view. When the light-emitting layer is grown on the growth substrate, only partial surface of the wafer 204 is contacted with the bottom surface 203 of the opening 202 of the wafer carrier 20. The wafer 204 is shaken easily and may fly out when the wafer carrier 20 is rotated at high speed.
FIG. 3A illustrates a conventional wafer carrier 30. The wafer carrier 30 comprises a carrier body 300 having an opening 302, wherein a bottom surface 303 of the opening 302 is a flat surface; and a supporting ring 305 provided around a periphery of the carrier body 300. A wafer 304 comprises a growth substrate and an epitaxial layer grown on the growth substrate, and the epitaxial layer comprises a light-emitting layer.
FIG. 3B illustrates a top view of the conventional wafer carrier 30. As illustrated in FIG. 3B, the top view of the supporting ring 305 is approximately a circular shape. The supporting ring 305 supports the wafer 304 around a periphery of the wafer 304 and the wafer 304 is not shaken easily. But the temperature of the wafer periphery contacted directly with the supporting ring 305 and the temperature of the wafer center not directly contacted with the supporting ring 305 are different, which results in different growth temperatures in different regions of the wafer 304 when the light-emitting layer is grown on the growth substrate.